The present invention relates to the use of a supported noble metal catalyst for preparing olefinically unsaturated carbonyl compounds from olefinically unsaturated alcohols by oxidative dehydrogenation, and to corresponding supported noble metal is catalysts.
More particularly, the present invention relates to the use of supported noble metal catalysts obtainable by a particular process for preparing 3-methylbut-2-en-1-al from 3-methylbut-3-en-1-ol.
The preparation of alpha,beta-unsaturated carbonyl compounds by oxidative dehydrogenation over suitable catalysts is known to those skilled in the art and has been described many times in the literature.
DE-B-20 20 865 accordingly describes a process for preparing alpha,beta-unsaturated carbonyl compounds, wherein the dehydrogenation catalysts used, according to the description, may be alloys and metal compounds, specifically some metal oxides of the transition group elements. It is additionally stated in this document that the catalysts can be used in pure form, such as in the form of mixed catalysts with or without support substance. Particularly suitable catalysts mentioned are zinc oxide, cadmium oxide and manganese oxide, and also mixed catalysts composed of the metals Cu, Ag and/or Zn. Regarding the preparation of the catalyst, there is no further information in this document.
EP-A 881 206 describes a process for continuous industrial preparation of unsaturated aliphatic aldehydes in a tube bundle reactor. Preferred catalysts mentioned for this process are supported silver catalysts which consist of spheres of an inert support material coated with 0.1 to 20% by weight, based on the amount of the support, of a layer of metallic silver in the form of a smooth, abrasion-resistant shell. In addition, a particular ratio of the greatest diameter of the coated catalyst spheres to the internal diameter of the reaction tube should preferably be observed.
DE-A 27 15 209 discloses a process for preparing 3-alkylbuten-1-als, wherein a catalyst with a total layer thickness of 5 to 35 mm and 2 or more layers of silver and/or copper crystals is used. The production of the catalyst with a plurality of layers is of the noble metal is relatively complex.
EP-A 357 292 discloses a process for preparing ethylene oxide. The catalysts used in this process are silver catalysts, wherein the silver has been applied to a porous heat-resistant support with a particular specific BET surface area. According to the information in this document, the silver can be applied to the support as a suspension of silver or silver oxide in a liquid medium, for example water, or by impregnation of the support with a solution of a silver compound. Subsequently, this silver compound is reduced to elemental silver by thermal treatment. There is no information in this document regarding a possible use of the supported silver catalysts thus prepared for preparing ethylenically unsaturated carbonyl compounds.
3-Methylbut-2-en-1-al, also known by the trivial name prenal, is an important precursor for citral, which is in turn an important product for a multitude of chemical syntheses. The catalysts described in the literature for preparation of prenal (3-methylbut-2-en-1-al) are produced by relatively complex processes and under production conditions which are in need of improvement overall. It would therefore be desirable to obtain supported noble metal catalysts for the synthesis of prenal from isoprenol (3-methylbut-3-en-1-ol), which are obtainable in a simple manner and which can also be controlled with regard to their selectivity simply by additions of compounds which act as promoters.